1. Field of the Invention
The present disclosure relates to binary or higher order high-density thermodynamically stable nanostructured metallic copper-based metallic systems, such as copper-tantalum (Cu-Ta) metallic systems, and methods of making the same.
2. Description of the Related Art
Bulk nanocrystalline metals, alloys, and composites have recently generated great interest and attention in the scientific community. This is mainly due to the exotic mechanical properties with which they are associated. Recent reports indicate that ultra-high strength and moderate ductility are possible in such metals. The combined possibility of ultra-high strength and ductility (i.e., ultra-tough nanocrystalline materials) make nanocrystalline metals and alloys the future of advanced metallurgy.
However, a major drawback to commercialization of these unique materials is the inability to mass produce large quantities of bulk material. Currently, commercialized products have been limited to electrolytic coatings and/or steels where the spacing of the microstructual phases is on the nanometer scale.
There are primarily two main methodologies for fabricating and producing nanocrystalline alloys. The two approaches available are a top-down approach and a bottom-up approach. In the top down processing approach, one takes a bulk piece of metal or alloy and through subjecting it to severe plastic deformation, the internal coarse grain size (tens of micrometers) of the bulk object is reduced to the nanoscale.
Top-down methods include equal channel angular extrusion (ECAE) or pressing, high pressure torsion (HPT), surface mechanical attrition treatment (SMAT), etc. Some of the top-down approaches suffer from limitations in the size and geometry of the materials which could be produced. For instance, in ECAE, the forces required to extrude a large billet are determined by its cross-sectional dimensions and could be exceedingly high if a large extrudate is desired. Additionally, due to the nature of the extrusion process, the fully deformed or worked region, especially during multi-pass extrusions, can be quite limited. Similarly, in HPT, because of the necessary pressures and confinement required, only relatively small 10- to 20-millimeter diameter by a few millimeters thick specimens can be fabricated. Likewise, in SMAT coatings, only the top few hundreds of micrometers beneath the exterior surface becomes deformation-processed having a nanostructure.
In contrast, the bottom-up approach entails the use of methods in which metallic particulates are produced. Typically, the particles have an average diameter of 10 nm to tens of millimeters. However, it is important to recognize that the larger particles still maintain an interior nanostructure despite their seemingly large size. There are multiple bottom up approaches including mechanical milling/alloying which could be used to produce a range of metallic particulates. Such bottom up processes used to produce nanostructured and nanocrystalline metals can be scaled-up readily to produce large quantities of powder.
Particulate (powdered) materials offer greater versatility when considering up-scaling to production and manufacturing levels. In part, this is because powder metallurgy is already a long term and existing practice being used to produce many commercially available products through sintering and forging of metallic particles into fully dense objects. Sintering is a method which allows for the production of near-net-shape, ready-to-use parts having almost unlimited dimensional restrictions while reducing the cost of post-production machining. While sintering functions to consolidate the loose particulates into a coherent solid, fully dense body, post-sinter forging is designed to impart the densified part with further increases in properties such as strength, ductility, etc.
Generally, in fine particulate materials, especially those with nano- to submicrometer size, there is an extremely large driving force to reduce the relative ratio of surface to volume area or surface to volume energy. This driving force is thermally activated and, therefore, occurs more efficiently at higher temperatures. The movement of particle boundaries, causes fine particles coalesce, merge, and grow into larger particles. If the temperature is near or in excess of 50% of the melting point of the material, this process is referred to as sintering. In addition to heat, if pressure could be applied to improve the sintering process, more rapid densification would occur, eliminating voids between the particles. If diffusion distances could be kept at a minimum, uninterrupted species transport could then be allowed. While some of the coarsening can be controlled by careful adjustment and selection of sintering conditions (i.e., an optimization and manipulation of the three dimensional processing surface of time, temperature, and pressure), the coarsening is unavoidable.
It should be clear that by nature, nanocrystalline or nanostructured powders tend to be metastable; that is, thermodynamically they are not in their lowest energy or the ground state, but instead, are in an elevated or higher energy state. As such, when favorable conditions arise, and energy may be released, thereby returning the material into its ground state, they coarsen to micrometer- or larger scale rapidly, even below conventional sintering temperatures. Thus, the coarsening or grain growth process with the concomitant reduction of the surface area to volume ratio returns the material to a lower energy state. Obviously, an associated effect of the coarsening process is the loss of the nano-grain size or nanostructure and the corresponding advantageous physical properties of the precursor powders. Therefore, while the powder metallurgically fabricated part is superior to conventionally produced equivalents, major improvements could still realized if the nanostructure could be retained in the product.
Schemes for preventing grain growth in nanocrystalline metals have been based on determining the velocity of grain boundaries undergoing curvature driven growth, which is the product of two terms: the mobility and the pressure of the grain boundaries. Therefore, two general approaches, namely, addressing each of these terms, independently, have been used to reduce grain growth in nanocrystalline metals. In the first, methods focus to modify the kinetics of grain growth by reducing the grain boundary mobility. In the second, methods are designed to modify the thermodynamics by reducing the driving force through attenuation of the grain boundary excess free energy which, in turn, decreases the driving pressure. Previous literature known to the inventors has shown both methods to be successful in preventing grain growth in some nanocrystalline systems. However, neither of these methodologies have been shown to be successful in the copper-tantalum (Cu-Ta) system. Specifically, the literature only speaks to the general aspects of thermodynamic stability; but, it does not speak directly to identifying the underlying and controlling thermodynamics involved in how to predict greater stability in Cu-Ta or other specific systems.